SE,B,8307139-9 (GKN-Stenman) describes a lock of this kind in which one of the intermediate pins has a larger diameter than corresponding upper pins in the first row of pin channels, and corresponding pin channels in the cylinder plug have a flared or widened part that is able to receive an intermediate pin of larger diameter. The intermediate pin can therewith be received in the widened part of said pin channel in the cylinder plug, but is unable to enter pin channels in the cylinder housing of smaller diameter, therewith latching the service key. Although this solution is simple from a technical aspect of lock manufacture, there is a danger that the intermediate pin will wedge firmly between a widened and a narrower channel part if an attempt is made to remove the service key when the lock is in its standard mode, therewith making it impossible to return the cylinder plug to the service mode. This, in turn, totally prevents removal of the service key, whereupon it becomes necessary to break open the lock in order to open the door.
NO,A,8007660-1 (Elkem-Spigerverket) describes another arrangement in which the number of pin channels in the standard mode differs from the number of pin channels in the service mode. Withdrawal of the service key in the normal mode is prevented by virtue of corresponding lower pins being unable to move upwards in this mode. However, it is relatively easy to grind down a relevant part of the key such as to avoid the latching effect.
SE,B,8800818-0 (Publication No. 460 797) (ASSA) describes a lock according to the preamble of claim 1, in other words the lock also includes a separate latching element that assists in preventing withdrawal of the service key in the standard mode of the lock. The latching element described in this document is disposed in the region between the inner end of the cylinder plug and the drive element, and will be subjected to comparatively large stresses and strains with subsequent wear in conjunction with use, and particularly in view of the necessary slenderness of the latching element. This presents the danger of a malfunction, therewith jeopardising the reliability of the lock system as a whole. For instance, when the plug is insufficiently displaced axially there is a greater risk that one or more pins will pass unintentionally into the pin channels in the plug and therewith prevent its rotation.